Here's why:
* Verb: "Brighten" means to make something brighter or to become brighter.
* Past participle: The past participle form of a verb is used to form perfect tenses (e.g., "has brightened", "had brightened") and passive voice (e.g., "the room was brightened").
Therefore, "brightened" can act as an adjective or part of a verb phrase, depending on the context.